This invention relates to an incremental angular encoder, i.e. a measurement sensor, which generates a digital signal representative of the instantaneous angular position of the shaft of the encoder.
There are two versions of angular encoders which emit their output signals in a digital format. One version is the so-called absolute angular encorder, which emits the angular information in a digital code, i.e. the angular value is immediately available in absolute form at any time, even after an operational interruption or a failure of the supply voltage of a system in which the angular encoder is installed. It is necessary to differentiate between such absolute angular encoders and an incremental angular encoder, which sub-divides one revolution of the angular encoder shaft, i.e. a rotational angle of 360.degree., into a defined number of steps--"increments".
The present invention concerns incremental angular encoders. In order to indicate the direction of rotation, the increment information in the form of binary signals is emitted on two different channels, i.e. at two separate connections, on a first channel A, and on a second channel B. Information regarding the direction of rotation can be derived by comparing the phase of the signal outputs on the two channels relative to one another. If, for example, channel A switches before channel B, the angular encoding shaft is rotating clockwise. If, on the other hand, channel B switches before channel A, the shaft is rotating counter-clockwise. The output signals on both channels are usually emitted in the form of binary signals. In many incremental angular encoders, a so-called reference signal is also supplied; this appears once per revolution, i.e. after each 360.degree. rotation of the shaft.
If such an incremental angular encoder is used in a system, for example as a distance measurement element perhaps in an industrial robot the output pulse signals which are generated must be counted by an electronic counting device in order to obtain information, for example on the distance traversed by the tool attached to the end of the arm of the robot. In contrast, when an absolute angular encoder is used, only a code converter is necessary because, as already mentioned, the absolute angular information is not lost even with a voltage interruption and there is a fixed relationship between the shaft angular position and the encoder output signal. One disadvantage of absolute angular encoders, however, is their high price, which becomes disproportionately higher with increasing encoder resolution. Another disadvantage is the increase in the size of the encoder as the resolution becomes greater. In contrast, the incremental angular encoder has the advantage of smaller physical size, particularly at high values of angular resolution, and a substantially lower price. However, an important disadvantage, which has already been discussed, is that after interruption and reconnection of the supply voltage, no information whatever is available on the angular position of the shaft and this is immediately lost again if the supply voltage fails--even briefly.
To remedy such problems, one prior art solution establishes a reference point. When the supply voltage is switched on, it is first necessary to drive the unit mechanically to this reference point so that the system can establish a zero point. This can be accomplished through the use of a reference signal, appearing once per revolution of the angular encoder shaft, and signals from proximity switches, end switches and the like. Such an approach involves expenditure on additional switches, which require additional wiring and adjustment work for installation and, unfortunately, can represent an additional source of error.
In most cases, incremental angular encoders are used to measure distances. A distance of X.sub.n, for example, is then represented by n output pluses or steps of the angular encoder. For this distance, several revolutions of the encoder shaft are generally necessary. It follows that the reference signal of the angular encoder, appearing every 360.degree., i.e. once per revolution of the shaft, cannot be used alone for determining the system zero point. Rather the reference signal must be used in conjunction with the sensor signals already mentioned to ensure a reproducible system zero point. Particularly in the manufacture of industrial robots, however, difficulties arise in generating this system zero point because the necessary sensor must be attached for example to the front tool--i.e. the hand--which means that the electrical signal must be carried by cable which is rotated and pivoted. In many cases, slip-rings must be used in the rotational or pivoted joints.